Re: CASS + DC = FAIL
When the Batgirl mini failed, DC blamed Cass. If this fails, they'll just blame Steph, with no thought of it being because Cass was out of the cowl. Then they'll probably go to putting BABS back in the cowl, not only ruining her character as Oracle, but making it pretty much 100% certain Cass will NEVER get her cowl back.
With Steph in the cowl, there is a better chance of Cass being able to come into the picture again (as they're best friends and Miller has already said Cass will show up again) and eventually even share the cowl (a Batgirl Corps thing) and maybe even eventually get it back if the writers decide to go with a Batgirl/Spoiler book.
Also, you may not be a Steph fan, but a lot of Cass fans are also Steph fans, including me. I (and many others) don't want to see one fail so another succeeds, which was why I was dissapointed in Cass leaving- but at this point, limbo might be the best thing for her, as pepperspray said. At least both characters are safe for now, and have possible good futures.